Adjustable-trim centrifugal compressor, and turbocharger having same

ABSTRACT

A centrifugal compressor for a turbocharger includes an inlet-adjustment mechanism in an air inlet for the compressor, operable to move between an open position and a closed position in the air inlet. The inlet-adjustment mechanism includes an axially elongated ring. In the open position, the radially outer surface of the ring is spaced from a tapering inner surface of the air inlet so that air can flow in an annular passage between the tapering surface and the ring. In the closed position, the ring abuts the tapering surface to close off the annular passage, whereby the effective inlet diameter is then defined by the inner diameter of the ring at its trailing edge. Movement of the inlet-adjustment mechanism from the open position to the closed position is effective to shift the compressor&#39;s surge line to lower flow rates.

BACKGROUND OF THE INVENTION

The present disclosure relates to centrifugal compressors, such as usedin turbochargers, and more particularly relates to centrifugalcompressors in which the effective inlet area or diameter can beadjusted for different operating conditions.

An exhaust gas-driven turbocharger is a device used in conjunction withan internal combustion engine for increasing the power output of theengine by compressing the air that is delivered to the air intake of theengine to be mixed with fuel and burned in the engine. A turbochargercomprises a compressor wheel mounted on one end of a shaft in acompressor housing and a turbine wheel mounted on the other end of theshaft in a turbine housing. Typically the turbine housing is formedseparately from the compressor housing, and there is yet another centerhousing connected between the turbine and compressor housings forcontaining bearings for the shaft. The turbine housing defines agenerally annular chamber that surrounds the turbine wheel and thatreceives exhaust gas from an engine. The turbine assembly includes anozzle that leads from the chamber into the turbine wheel. The exhaustgas flows from the chamber through the nozzle to the turbine wheel andthe turbine wheel is driven by the exhaust gas. The turbine thusextracts power from the exhaust gas and drives the compressor. Thecompressor receives ambient air through an inlet of the compressorhousing and the air is compressed by the compressor wheel and is thendischarged from the housing to the engine air intake.

Turbochargers typically employ a compressor wheel of the centrifugal(also known as “radial”) type because centrifugal compressors canachieve relatively high pressure ratios in a compact arrangement. Intakeair for the compressor is received in a generally axial direction at aninducer portion of the centrifugal compressor wheel and is discharged ina generally radial direction at an exducer portion of the wheel. Thecompressed air from the wheel is delivered to a volute, and from thevolute the air is supplied to the intake of an internal combustionengine.

The operating range of the compressor is an important aspect of theoverall performance of the turbocharger. The operating range isgenerally delimited by a surge line and a choke line on an operating mapfor the compressor. The compressor map is typically presented aspressure ratio (discharge pressure Pout divided by inlet pressure Pin)on the vertical axis, versus corrected mass flow rate on the horizontalaxis. The choke line on the compressor map is located at high flow ratesand represents the locus of maximum mass-flow-rate points over a rangeof pressure ratios; that is, for a given point on the choke line, it isnot possible to increase the flow rate while maintaining the samepressure ratio because a choked-flow condition occurs in the compressor.

The surge line is located at low flow rates and represents the locus ofminimum mass-flow-rate points without surge, over a range of pressureratios; that is, for a given point on the surge line, reducing the flowrate without changing the pressure ratio, or increasing the pressureratio without changing the flow rate, would lead to surge occurring.Surge is a flow instability that typically occurs when the compressorblade incidence angles become so large that substantial flow separationarises on the compressor blades. Pressure fluctuation and flow reversalcan happen during surge.

In a turbocharger for an internal combustion engine, compressor surgemay occur when the engine is operating at high load or torque and lowengine speed, or when the engine is operating at a low speed and thereis a high level of exhaust gas recirculation (EGR). Surge can also arisewhen an engine is suddenly decelerated from a high-speed condition.Expanding the surge-free operation range of a compressor to lower flowrates is a goal often sought in compressor design.

BRIEF SUMMARY OF THE DISCLOSURE

The present disclosure describes mechanisms and methods for acentrifugal compressor that can enable the surge line for the compressorto selectively be shifted to the left (i.e., surge is delayed to a lowerflow rate at a given pressure ratio). One embodiment described hereincomprises a turbocharger having the following features:

a turbine housing and a turbine wheel mounted in the turbine housing andconnected to a rotatable shaft for rotation therewith, the turbinehousing receiving exhaust gas and supplying the exhaust gas to theturbine wheel;

a centrifugal compressor assembly comprising a compressor housing and acompressor wheel mounted in the compressor housing and connected to therotatable shaft for rotation therewith, the compressor wheel havingblades and defining an inducer portion, the compressor housing definingan air inlet for leading air generally axially into the inducer portionof the compressor wheel, the compressor housing further defining avolute for receiving compressed air discharged generally radiallyoutwardly from the compressor wheel, the air inlet having an innersurface a portion of which defines a sliding surface that extends for anaxial length along a downstream axial direction, followed by a shroudsurface that is adjacent to outer tips of the blades of the compressorwheel; and

a compressor inlet-adjustment mechanism disposed in the air inlet of thecompressor housing and movable between an open position and a closedposition.

The inlet-adjustment mechanism comprises an axially elongated ring whoseinner surface becomes smaller in diameter in a downstream directiontoward the compressor wheel, the ring terminating at a trailing edge.The inner diameter of the ring at the trailing edge is smaller than aninner diameter of the shroud surface of the compressor housing at theinducer portion of the compressor wheel. The ring is arranged so thatwhen the inlet-adjustment mechanism is in the open position the trailingedge of the ring is axially spaced relatively farther upstream from theinducer portion such that an effective diameter of the air inlet at theinducer portion is determined by the shroud surface, and when theinlet-adjustment mechanism is in the closed position the trailing edgeof the ring is axially spaced relatively closer to the inducer portionof the compressor wheel such that the effective diameter of the airinlet at the inducer portion is determined by the inner diameter of thetrailing edge of the ring.

In some embodiments the air inlet further defines a tapering innersurface that follows the sliding surface and extends for an axial lengthalong the downstream axial direction, the tapering inner surfacebecoming smaller in diameter in the downstream axial direction. In suchembodiments the ring of the inlet-adjustment mechanism in the openposition is spaced from the tapering surface of the air inlet such thatthere is an annular passage between the tapering surface and the ringfor a flow of air therethrough, and when the inlet-adjustment mechanismis in the closed position the ring abuts the tapering surface toeliminate said annular passage.

The inlet-adjustment mechanism in one embodiment further comprises asupport portion that is joined by a plurality of circumferentiallyspaced struts to the ring. The support portion has a radially outersurface that engages the inner surface of the air inlet and is movablealong the inner surface either by axial sliding with no rotation or bycombined axial and rotational movement such as how a screw moves.

The struts can have any of various cross-sectional shapes, for example,an airfoil cross-sectional shape in a θ-z plane. The number of suchstruts, and their shapes and thicknesses, can be selected depending onthe needs of a particular situation. In one embodiment there are threeof the struts circumferentially spaced apart.

The ring can comprise a tubular wall having any of variouscross-sectional shapes, for example, an airfoil cross-sectional shape inan r-z plane.

The tubular wall at a trailing edge thereof can define an insidediameter that is between 0.45 and 0.98 times d_(1S), where d_(1S) is adiameter of the inducer portion of the compressor wheel.

The tubular wall has an axial length L and the trailing edge of thetubular wall in the closed position is spaced an axial distance S from aleading edge of the inducer portion of the compressor wheel. The lengthof the tubular wall should be relatively large in comparison with thespacing distance S.

The spacing distance S advantageously should be as small as practicable.For example, when S<8.5(d_(1S)−d_(r)), where d_(r) is the inner diameterof the ring at its trailing edge, the inlet-adjustment mechanism isexpected to have a beneficial effect on surge margin, the benefitgenerally becoming larger as S becomes smaller.

The ring can be formed substantially as a body of revolution having ashape like a funnel. In some embodiments the ring has no openingsthrough its wall. In other embodiments, the wall of the ring hasopenings that allow some flow through from the interior to the exteriorof the ring when the ring is in the open position, but the compressorhousing is configured to block the openings when the ring is in theclosed position.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING(S)

Having thus described the invention in general terms, reference will nowbe made to the accompanying drawings, which are not necessarily drawn toscale, and wherein:

FIG. 1 is a perspective view of a turbocharger, with a portion of thecompressor housing cut away to show internal details, in accordance withone embodiment of the invention;

FIG. 2 is an axial cross-sectional view of the turbocharger of FIG. 1,with the inlet-adjustment mechanism in the closed position;

FIG. 2A is an enlarged portion of FIG. 2;

FIG. 3 is a view similar to FIG. 2, but with the inlet-adjustmentmechanism in the open position;

FIG. 4 is an exploded view of the turbocharger of FIG. 1;

FIG. 5 is a perspective view of a turbocharger, with a portion of thecompressor housing cut away to show internal details, in accordance withanother embodiment of the invention;

FIG. 6 is an axial cross-sectional view of the turbocharger of FIG. 5,with the inlet-adjustment mechanism in the closed position;

FIG. 7 is a view similar to FIG. 6, but with the inlet-adjustmentmechanism in the open position;

FIG. 8 is an exploded view of the turbocharger of FIG. 5;

FIG. 9 is a perspective view of an inlet-adjustment ring in accordancewith a further embodiment of the invention;

FIG. 10 is a cross-sectional view of a turbocharger having the ring ofFIG. 9, in the open position; and

FIG. 11 is a view similar to FIG. 10, with the ring in the closedposition.

DETAILED DESCRIPTION OF THE DRAWINGS

The present inventions now will be described more fully hereinafter withreference to the accompanying drawings, in which some but not allembodiments of the inventions are shown. Indeed, these inventions may beembodied in many different forms and should not be construed as limitedto the embodiments set forth herein; rather, these embodiments areprovided so that this disclosure will satisfy applicable legalrequirements. Like numbers refer to like elements throughout.

A turbocharger 10 in accordance with one embodiment of the invention isillustrated in cross-sectional view in FIG. 2. The turbochargercomprises a compressor 12 having a compressor wheel or impeller 14mounted in a compressor housing 16 on one end of a rotatable shaft 18.The compressor housing defines an air inlet 17 for leading air generallyaxially into the compressor wheel 14. The shaft 18 is supported inbearings 19 mounted in a center housing 20 of the turbocharger. Theshaft 18 is rotated by a turbine wheel 22 mounted on the other end ofthe shaft 18 from the compressor wheel, thereby rotatably driving thecompressor wheel, which compresses air drawn in through the compressorinlet and discharges the compressed air generally radially outwardlyfrom the compressor wheel into a volute 21 for receiving the compressedair. From the volute 21, the air is routed to the intake of an internalcombustion engine (not shown) for boosting the performance of theengine.

The air inlet 17 has an inner sliding surface 17 s that extends for anaxial length along a downstream axial direction, followed by a taperinginner surface 17 t that extends for an axial length along the downstreamaxial direction. The tapering inner surface becomes smaller in diameterin the downstream axial direction.

The compressor housing 16 defines a shroud surface 16 s that is closelyadjacent to the radially outer tips of the compressor blades. The shroudsurface 16 s defines a curved contour that is generally parallel to thecontour of the compressor wheel. At the inlet to the inducer portion 14i of the compressor wheel, the shroud surface 16 s has a diameter thatis slightly greater than the diameter d_(1S) of the inducer portion 14i.

The turbocharger further includes a turbine housing 24 that houses theturbine wheel 22. The turbine housing defines a generally annularchamber 26 that surrounds the turbine wheel and that receives exhaustgas from the internal combustion engine for driving the turbine wheel.The exhaust gas is directed from the chamber 26 generally radiallyinwardly through a turbine nozzle 28 to the turbine wheel 22. As theexhaust gas flow through the passages between the blades 30 of theturbine wheel, the gas is expanded to a lower pressure, and the gasdischarged from the wheel exits the turbine housing through a generallyaxial bore 32 therein.

The turbine nozzle 28 is a variable nozzle for varying thecross-sectional flow area through the nozzle so as to regulate flow intothe turbine wheel. The nozzle includes a plurality of vanes 34 that arecircumferentially spaced about the nozzle. Each vane is affixed to anaxle (not shown) that passes through an aperture in a generally annularnozzle ring 38 that is mounted coaxially with respect to the turbinewheel 22. Each axle is rotatable about its axis for rotating theattached vane. The nozzle ring 38 forms one wall of the flow passage ofthe nozzle 28. Each of the axles has a vane arm (not specificallyillustrated) affixed to an end of the axle that protrudes out from thenozzle ring 38, and is engaged by a generally annular unison ring 42(also referred to herein as an actuator ring) that is rotatable aboutits axis and that is coaxial with the nozzle ring 38. An actuator (notshown) is connected to the unison ring 42 for rotating it about itsaxis. When the unison ring is rotated, the vane arms are rotated tocause the axles to rotate about their axes, thereby rotating the vanes34 so as to vary the cross-sectional flow area through the nozzle 28. Asdescribed thus far, the variable nozzle mechanism generally correspondsto a conventional variable nozzle having variable vanes.

In the illustrated embodiment, the variable vane mechanism is providedin the form of a cartridge 50 that is installable into and removablefrom the turbocharger as a unit. The cartridge 50 comprises the nozzlering 38, vanes 34, axles, vane arms, and unison ring 42. The cartridgefurther comprises an insert 52 that has a tubular portion 54 sealinglyreceived into a portion 32 a of the bore 32 of the turbine housing, anda nozzle portion 56 extending generally radially out from one end of thetubular portion 54, the nozzle portion 56 being axially spaced from thenozzle ring 38 such that the vanes 34 extend between the nozzle ring 38and the nozzle portion 56. The bore portion 32 a of the turbine housinghas a radius that exceeds that of the remainder of the bore 32 by anamount slightly greater than the radial thickness of the tubular portion54 of the insert 52. The radially outer surface of the tubular portion54 has at least one circumferential groove, and preferably has twoaxially spaced grooves as shown in FIG. 2, in each of which a sealingring 58 is retained for sealingly engaging the inner surface of the boreportion 32 a. Advantageously, the outer diameter of the tubular portion54 of the insert is slightly less than the inner diameter of the boreportion 32 a so that a slight gap is defined therebetween, and only thesealing rings 58 make contact with the inner surface of the bore portion32 a. Additionally, there is a gap 60 between the nozzle portion 56 andthe adjacent end of the turbine housing at the end of the bore portion32 a. In this manner, the insert 52 is mechanically and thermallydecoupled from the turbine housing 24.

A plurality of spacers 70 are connected between the nozzle ring 38 andthe nozzle portion 56 of the insert 52 for securing the nozzle ring tothe insert and maintaining the desired axial spacing between the nozzleportion of the insert and the nozzle ring.

The cartridge 50 further comprises a heat shroud 80 that is captivelyretained between the nozzle ring 38 and the center housing 20 when thecartridge is installed onto the center housing. The heat shroud 80provides sealing between the nozzle ring and center housing to preventhot exhaust gas from migrating between these parts into the cavity inwhich the vane arms and unison ring 42 are disposed. The heat shroud 80advantageously is a resiliently elastic material such as spring steel orthe like, and the shroud is configured so that it is compressed in theaxial direction between the nozzle ring 38 and the center housing 20 sothat the restoring force of the shroud urges the nozzle ring axially (tothe right in FIG. 2) against a flange or retainer 82 that is sandwichedbetween the center housing and the turbine housing, thereby axiallylocating the nozzle ring (and thus the entire variable nozzle cartridge50) within the turbocharger. In this regard the cartridge 50 is axiallylocated in substantially the same way described in commonly owned U.S.Pat. No. 8,333,556, the entire disclosure of which is herebyincorporated herein by reference. The cartridge is radially located by alocator ring 84 whose radially outer periphery engages a radiallyinwardly facing surface of the nozzle ring 38 and whose radially innerperiphery engages a radially outwardly facing surface of the centerhousing 20.

In accordance with the invention, the compressor of the turbochargerincludes an inlet-adjustment mechanism 100 disposed in the air inlet 17of the compressor housing and movable (either axially slidable orhelicoidally movable in the manner of a screw) between an open position(FIG. 3) and a closed position (FIGS. 2 and 2A). The inlet-adjustmentmechanism comprises an axially elongated ring 110. The ring 110 isarranged so that when the inlet-adjustment mechanism 100 is in the openposition (FIG. 3) the ring 110 is spaced from the tapering surface 17 tof the air inlet such that there is an annular passage 112 between thetapering surface and the ring for a flow of air therethrough. When theinlet-adjustment mechanism 100 is in the closed position (FIGS. 2 and2A) the ring 110 abuts the tapering surface 17 t to eliminate theannular passage 112.

The ring 110 in the closed position (FIGS. 2 and 2A) extends in thedownstream direction substantially to the inducer 14 i of the compressorwheel 14, such that an effective diameter of the air inlet at theinducer portion is determined by the inner diameter d_(r) of the ring110 at its trailing edge.

The inlet-adjustment mechanism 100 in the illustrated embodiment furthercomprises a support portion that includes a plurality ofcircumferentially spaced struts 114 that connect to the ring 110. Thesupport portion has a radially outer surface that engages the slidingsurface 17 s of the air inlet 17 and is movable along the slidingsurface. In the illustrated embodiment the outer surface of the supportportion is defined by outer surfaces of the struts 114. Alternatively,however, the support portion could include a further portion (e.g., aring or the like) connected to the struts 114, and such further portioncould define the outer surface that slides along the cylindrical surfaceof the inlet.

The struts 114 can have any of various cross-sectional shapes, forexample, an airfoil cross-sectional shape in a θ-z plane. The struts canbe various in number. The illustrated embodiment has four struts, butother numbers can be used instead.

With reference to FIG. 2A, the ring 110 comprises a tubular wall thatcan have any of various cross-sectional shapes, for example, an airfoilcross-sectional shape in an r-z plane. The tubular wall at a trailingedge thereof defines an inside diameter d_(r) that is between 0.45 and0.98 times d_(1S), where d_(1S) is a diameter of the inducer portion 14i of the compressor wheel.

The tubular wall has an axial length L and the trailing edge of thetubular wall in the closed position is spaced an axial distance S from aleading edge of the inducer portion of the compressor wheel. The lengthof the tubular wall should be relatively large in comparison with thespacing distance S.

The spacing distance S advantageously should be as small as practicable.For example, when S<8.5(d_(1S)−d_(r)), the inlet-adjustment mechanism isexpected to have a beneficial effect on surge margin, the benefitgenerally becoming larger as S becomes smaller. When S is larger thanthat value, it is expected that the benefit on surge margin will benegligible or nonexistent. This rule of thumb, however, is based on alimited amount of investigations in a particular turbochargerconfiguration, and thus cannot be viewed as a rule that applies to allturbocharger configurations. Accordingly, not all of the appended claimsare restricted to S values abiding by this rule of thumb.

The inlet-adjustment mechanism 100 enables adjustment of the effectivesize or diameter of the inlet into the compressor wheel 14. Asillustrated in FIGS. 2 and 2A, when the inlet-adjustment mechanism is inthe closed position, the effective diameter of the inlet into thecompressor wheel is dictated by the inside diameter d_(r) of the ring110 at its trailing edge. In order for this effect to be achieved, theaxial spacing distance S must be as small as practicable, as previouslydescribed, so that the entire airflow into the compressor wheel passesthrough the interior of the ring 110 and there is insufficient distancedownstream of the ring's trailing edge for the flow to expand to thefull diameter of the inducer portion 14 i of the compressor wheel 14 bythe time the air encounters it. The inlet diameter is therebyeffectively reduced to a value that is dictated by the ring diameterd_(r).

On the other hand, when the inlet-adjustment mechanism 100 is moved tothe open position of FIG. 3, some portion of the air entering the inlet17 is able to flow through the annular space 112 between the ring 110and the inner surface of the inlet, and thus the effective diameter ofthe inlet is the full diameter of the inlet (as defined by the shroudsurface 16 s) at the inducer portion 14 i.

At low flow rates (e.g., low engine speeds), the inlet-adjustmentmechanism 100 can be placed in the closed position of FIGS. 2 and 2A.This can have the effect of reducing the effective inlet diameter andthus of increasing the flow velocity into the compressor wheel. Theresult will be a reduction in compressor blade incidence angles,effectively stabilizing the flow (i.e., making blade stall andcompressor surge less likely). In other words, the surge line of thecompressor will be moved to lower flow rates (to the left on a map ofcompressor pressure ratio versus flow rate).

At intermediate and high flow rates, the inlet-adjustment mechanism 100can be opened as in FIG. 3. This can have the effect of increasing theeffective inlet diameter so that the compressor regains its high-flowperformance and choke flow essentially as if the inlet-adjustmentmechanism were not present and as if the compressor had a conventionalinlet matched to the wheel diameter at the inducer portion of the wheel.

A second embodiment of the invention is illustrated in FIGS. 5-8. Theturbocharger 10′ of the second embodiment is generally similar to thefirst embodiment described above, except for the configuration of theinlet-adjustment mechanism 100′. The inlet-adjustment mechanism 100′ ofthe second embodiment comprises an axially elongated ring 120 that lacksthe struts of the previous embodiment. Instead, the ring 120 is formedsubstantially as a body of revolution having a shape like a funnel. Whenthe ring 120 is in the closed position of FIGS. 5 and 6, the ring abutsthe tapering surface 17 t of the compressor inlet and therefore all ofthe air must flow through the interior of the ring. Accordingly, becausethe ring in this position is closely adjacent to the inducer portion ofthe compressor wheel (i.e., the spacing distance S in FIG. 6 isrelatively small), the effective inlet diameter at the inducer portionis dictated by the inner diameter d_(r) of the ring at its trailingedge, which is substantially smaller than the diameter of the shroudsurface 16 s at the inducer.

On the other hand, when the ring 120 is slid to the open position ofFIG. 7, there is a substantial axial spacing between the trailing edgeof the ring and the inducer portion 14 i of the compressor wheel. As aresult, even though all of the flow still passes through the interior ofthe ring (because the ring engages the inner surface of the cylindricalportion 17 s of the air inlet over a full 360 degrees), there issufficient distance from the end of the ring to the inducer portion toallow the flow to re-expand to a larger diameter before reaching theinducer portion 14 i. Accordingly, the effective inlet diameter isnearly the full inlet diameter at the inducer. If the inner diameter ofthe ring 120 is too small such that the flow chokes in the orifice undercertain conditions, a bypass solution can be implemented, as describedbelow in connection with a further embodiment.

Such a further embodiment of the invention is illustrated in FIGS. 9-11.This embodiment is substantially similar to the embodiment of FIGS. 5-8,except for the configuration of the ring. In the further embodiment, thering 120′ is a body of revolution shaped like a funnel, but it includesa number of openings 122 through the wall of the ring to allow air topass through under certain conditions. When the ring is in the openposition as in FIG. 10, the majority of the air will flow through theinterior of the ring as in the previous embodiment, but some air willflow through the openings 122 from the interior to the exterior of thering before the air encounters the inducer portion 14 i of thecompressor wheel. This helps facilitate a substantially complete fillingof the entire air inlet just upstream of the inducer portion 14 i. Whenthe ring is in the closed position of FIG. 11, the tapering surface 17 tof the compressor housing closes off the openings 122 so that air issubstantially prevented from passing through the openings, and hence allof the air must flow through the interior of the ring 120′.

Many modifications and other embodiments of the inventions set forthherein will come to mind to one skilled in the art to which theseinventions pertain having the benefit of the teachings presented in theforegoing descriptions and the associated drawings. Therefore, it is tobe understood that the inventions are not to be limited to the specificembodiments disclosed and that modifications and other embodiments areintended to be included within the scope of the appended claims.Although specific terms are employed herein, they are used in a genericand descriptive sense only and not for purposes of limitation.

What is claimed is:
 1. A turbocharger, comprising: a turbine housing anda turbine wheel mounted in the turbine housing and connected to arotatable shaft for rotation therewith, the turbine housing receivingexhaust gas and supplying the exhaust gas to the turbine wheel; acentrifugal compressor assembly comprising a compressor housing and acompressor wheel mounted in the compressor housing and connected to therotatable shaft for rotation therewith, the compressor wheel havingblades and defining an inducer portion, the compressor housing definingan air inlet for leading air generally axially into the compressorwheel, the compressor housing further defining a volute for receivingcompressed air discharged generally radially outwardly from thecompressor wheel, the air inlet having a an inner surface a portion ofwhich comprises a sliding surface that extends for an axial length alonga downstream direction, followed by a shroud surface that is adjacent toouter tips of the blades of the compressor wheel; a compressorinlet-adjustment mechanism disposed in the air inlet of the compressorhousing and movable between an open position and a closed position, theinlet-adjustment mechanism comprising an axially elongated ring whoseinner surface becomes smaller in diameter in the downstream directiontoward the compressor wheel, the ring terminating at a trailing edge,the inner diameter of the ring at the trailing edge being smaller thanan inner diameter of the shroud surface of the compressor housing at theinducer portion of the compressor wheel, wherein the ring is arranged sothat when the inlet-adjustment mechanism is in the open position thetrailing edge of the ring is axially spaced relatively farther upstreamfrom the inducer portion such that an effective diameter of the airinlet at the inducer portion is determined by the shroud surface, andwherein the trailing edge of the ring in the closed position is axiallyspaced relatively closer to the inducer portion of the compressor wheelsuch that the effective diameter of the air inlet at the inducer portionis determined by the inner diameter of the ring at the trailing edge. 2.The turbocharger of claim 1, wherein the air inlet further defines atapering inner surface that is axially disposed between the slidingsurface and the shroud surface and extends for an axial length along thedownstream direction, the tapering inner surface becoming smaller indiameter in the downstream axial direction, and wherein the ring of theinlet-adjustment mechanism in the open position is spaced from thetapering surface of the air inlet such that there is an annular passagebetween the tapering surface and the ring for a flow of airtherethrough, and when the inlet-adjustment mechanism is in the closedposition the ring abuts the tapering surface to eliminate said annularpassage.
 3. The turbocharger of claim 2, wherein the inlet-adjustmentmechanism further comprises a support portion that is joined by aplurality of circumferentially spaced struts to the ring, the supportportion having a radially outer surface that engages the cylindricalinner surface of the air inlet and is movable along the cylindricalinner surface.
 4. The turbocharger of claim 3, wherein the struts havean airfoil cross-sectional shape in a θ-z plane.
 5. The turbocharger ofclaim 3, wherein there are four said struts circumferentially spacedapart.
 6. The turbocharger of claim 3, wherein the ring comprises atubular wall having an airfoil cross-sectional shape in an r-z plane. 7.The turbocharger of claim 1, wherein the inside diameter of the ring atthe trailing edge is between 0.45 and 0.98 times d_(1S), where d_(1S) isa diameter of the inducer portion of the compressor wheel.
 8. Theturbocharger of claim 7, wherein the tubular wall has an axial length Land the trailing edge of the tubular wall in the closed position isspaced an axial distance S from a leading edge of the inducer portion ofthe compressor wheel, and wherein L is substantially larger than S. 9.The turbocharger of claim 8, wherein S<8.5(d_(1S)−d_(r)).
 10. Theturbocharger of claim 1, wherein the ring is formed by a wall configuredsubstantially as a body of revolution having a funnel shape.
 11. Theturbocharger of claim 10, wherein the wall of the ring is free of anyopenings through the wall.
 12. The turbocharger of claim 10, wherein thewall of the ring includes openings through the wall to allow air to passthrough the openings.
 13. The turbocharger of claim 12, wherein theopenings in the ring are unblocked when the ring is in the open positionso that air passes through the openings from an interior to an exteriorof the ring before encountering the inducer portion of the compressorwheel.
 14. The turbocharger of claim 13, wherein the compressor housingis configured to block the openings when the ring is in the closedposition so that air is substantially prevented from passing through theopenings.